1. Technical Field
The present invention generally relates to a process for preparing an overbased salt of a sulfurized alkyl-substituted hydroxyaromatic composition.
2. Description of the Related Art
The lubricant additive industry generally uses alkyl phenols (e.g., tetrapropenyl phenol, TPP) to prepare detergents comprising sulfurized metal alkyl phenates. Metal salts of sulfurized alkylphenols are useful lubricating oil additives which impart detergency and dispersancy properties to the lubricating oil composition for marine, automotive, railroad and air-cooled engines as well as providing for an alkalinity reserve in the oil. Alkalinity reserve is necessary in order to neutralize acids generated during engine operation. Without this alkalinity reserve, the acids so generated would result in harmful engine corrosion.
In the not so distant past, rapidly escalating energy costs, particularly those incurred in distilling crude oil and liquid petroleum, became burdensome to the users of transportation fuels, such as owners and operators of seagoing ships. In response, those users have steered their operations away from steam turbine propulsion units in favor of large marine diesel engines that are more fuel efficient. Diesel engines may generally be classified as slow-speed, medium-speed, or high-speed engines, with the slow-speed variety being used for the largest, deep shaft marine vessels and certain other industrial applications.
Slow-speed diesel engines are unique in size and method of operation. The engines themselves are massive, the larger units may approach 200 tons in weight and an upward of 10 feet in length and 45 feet in height. The output of these engines can reach as high as 100,000 brake horsepower with engine revolutions of 60 to about 200 revolutions per minute. They are typically of crosshead design and operate on the two-stroke cycle.
Medium-speed engines, on the other hand, typically operate in the range of about 250 to about 1100 rpm and may operate on either the four-stroke or the two-stroke cycle. These engines can be of trunk piston design or occasionally of crosshead design. They typically operate on residual fuels, just like the slow-speed diesel engines, but some may also operate on distillate fuels that contain little or no residue. These engines can also be used for propulsion, ancillary applications or both on deep-sea vessels.
Slow- and medium-speed diesel engines are also extensively used in power plant operations. A slow- or medium-speed diesel engine that operates on the 2-stroke cycle is typically a direct-coupled and direct-reversing engine of crosshead construction, with a diaphragm and one or more stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil. The notable complete separation of the crankcase from the combustion zone has led persons skilled in the art to lubricate the combustion chamber and the crankcase with different lubricating oils.
Accordingly, in large diesel engines of the crosshead type used in marine and heavy stationary applications, the cylinders are lubricated separately from the other engine components. The cylinders are lubricated on a total loss basis with the cylinder oil being injected separately to quills on each cylinder by means of lubricators positioned around the cylinder liner. Oil is distributed to the lubricators by means of pumps, which are, in modern engine designs, actuated to apply the oil directly onto the rings to reduce wastage of the oil.
The high stresses encountered in these engines and the use of residual fuels creates the need for lubricants with a high detergency and neutralizing capability even though the oils are exposed to thermal and other stresses only for short periods of time. Residual fuels commonly used in these diesel engines typically contain significant quantities of sulfur, which, in the combustion process, combine with water to form sulfuric acid, the presence of which leads to corrosive wear. In particular, in two-stroke engines for ships, areas around the cylinder liners and piston rings can be corroded and worn by the acid. Therefore, it is important for diesel engine lubricating oils to have the ability to resist such corrosion and wear.
Accordingly, a primary function of marine cylinder lubricants is to neutralize sulfur-based acidic components of high-sulfur fuel oil combusted in slow-speed 2-cycle crosshead diesel engines. This neutralization is accomplished by the inclusion in the marine cylinder lubricant of basic species such as metallic detergents, e.g., sulfurized metal alkyl phenates. Unfortunately the basicity of the marine cylinder lubricant can be diminished by oxidation of the marine cylinder lubricant (caused by the thermal and oxidative stress the lubricant undergoes in the engine), thus decreasing the lubricant's neutralization ability. The oxidation can be accelerated if the marine cylinder lubricants contain oxidation catalysts such as wear metals that are generally known to be present in the lubricant during engine operation.
Medium-speed trunk piston engines typically operate using various types and qualities of diesel fuels and heavy fuel oils. These engines are lubricated with trunk piston engine oils which are required to have the ability to form a protective layer between moving surfaces, neutralize acids, and keep contaminants suspended in the oil. Unfortunately, these properties can be adversely affected by oxidation of the oil resulting in viscosity increase, loss of neutralization capacity and loss of detergency. Accordingly, there is a need for improved detergents such as sulfurized metal alkyl phenates which provide better oxidative stability to a lubricating oil composition such as marine diesel engine lubricating oil compositions, e.g., marine cylinder lubricants and trunk piston engine oils.
The lubricant additive industry generally uses alkyl phenols (e.g., tetrapropenyl phenol, TPP) to prepare detergents comprising sulfurized metal alkyl phenates. Metal salts of sulfurized alkylphenols are useful lubricating oil additives which impart detergency and dispersancy properties to the lubricating oil composition for marine, automotive, railroad and air-cooled engines as well as providing for an alkalinity reserve in the oil. Alkalinity reserve is necessary in order to neutralize acids generated during engine operation. Without this alkalinity reserve, the acids so generated would result in harmful engine corrosion. However, there may be some unreacted alkyl phenols such as tetrapropenyl phenol present in the sulfurized metal alkyl phenate as well as in lubricating oils containing one or more of the sulfurized metal alkyl phenates.
A recent reproductive toxicity study in rats sponsored by the Petroleum Additives Panel of the American Chemistry Council shows that free or unreacted TPP may cause adverse effects on male and female reproductive organs. Further, it is believed that TPP may be corrosive or irritating to the skin.
U.S. Patent Application Publication No. 20080070818 (“the '818 publication”) discloses a lubricating oil composition including at least one sulfurized overbased metal phenate detergent prepared from a C9-C15 alkyl phenol, at least one sulfurizing agent, at least one metal and at least one overbasing agent; the detergent including less than 6.0% by combined mass of unsulfurized C9-C15 alkyl phenol and unsulfurized metal salts thereof.
U.S. Patent Application Publication No. 20090143264 (“the '264 publication”) discloses sulfurized metal alkyl phenate compositions having a low alkyl phenol content. The sulfurized metal alkyl phenate compositions of the '264 publication can be prepared by reacting a phenol compound such as tetrapropenyl phenol with an aldehyde to form a phenolic resin and then reacting the phenolic resin simultaneously with a metal base and a first sulfurizing agent.
U.S. Pat. No. 4,328,111 (“the '111 patent”) discloses that overbased phenates, including sulfurized phenates are commonly manufactured in the presence of ethylene glycol which is difficult to remove from the product, thereby wasting raw materials and sometimes leading to undesirable side effects from glycol in the final product. The '111 patent further discloses that in order to remove ethylene glycol, an acidic compound is reacted with a basic compound comprising an overbased metal sulfonate, phenate, or mixtures thereof, and the reaction product is then nitrogen stripped to remove the ethylene glycol.
A need still remains, therefore, for an improved overbased salt of a sulfurized alkyl-substituted hydroxyaromatic composition having oxidative stability in a lubricating oil composition such as, for example, a marine diesel engine lubricating oil composition. In addition, to reduce any potential health risks to customers and to avoid potential regulatory issues, there is a need to reduce the amount of free unsulfurized alkyl-substituted hydroxyaromatic compound and its metal salt in the salt of a sulfurized alkyl-substituted hydroxyaromatic composition in a simple, cost efficient manner. Accordingly, it is also desirable to provide an improved process for preparing a salt of a sulfurized alkyl-substituted hydroxyaromatic composition which has relatively low levels of unsulfurized alkyl substituted hydroxyaromatic compound and its metal salt.